1. Field of the Invention
This invention generally relates to wireless telephone communications and, more particularly, to a system and method for managing Candidate Frequency Searches in communications between wireless communications telephone devices and wireless communications base stations.
2. Description of the Related Art
A wireless communications telephone base station (BS) provides a serving frequency (band) for a Code Division Multiple Access (CDMA) wireless communications device within a coverage area of limited size. As a wireless device moves through one coverage area to an adjoining coverage area, the wireless device switches from the serving frequency for the first coverage area BS to the serving frequency for the adjoining coverage area BS. Such a switch is called a hard handoff. For example, a wireless device moves to the edge of a first coverage area until the power level for the serving frequency in the first area is insufficient to be received by the wireless device. At that point, the first BS instructs the wireless device to switch to the BS for an adjoining coverage area with a serving frequency having a power level sufficient to be received by the wireless device. If a problem occurs during the switch to the adjoining serving frequency, the call may be dropped since the first serving frequency is no longer supporting the call.
Hard handoffs can be performed by the BS without assistance from the wireless device. In these instances, a BS monitors the signal strength from a wireless device, and when the observed power level is not sufficient to support communications between the BS and the wireless device, the BS issues an order to the wireless device to switch or handoff to a different serving frequency corresponding to an adjacent coverage area. Unassisted hard handoffs have at least two problems. Frequently, calls are dropped while performing a hard handoff, because the wireless device has insufficient signal quality from the adjacent coverage area. In other cases, the wireless device ping-pongs, or oscillates between the serving frequency in the first coverage area and the serving frequency in an adjoining coverage area. This occurs if the power level from the wireless device observed by the BS in the adjoining area is not consistently sufficient to support communications between the BS and the wireless device.
With the advent of TIA/EIA-95-B, hard handoffs can be performed with the help of the wireless device. Such handoffs are called Mobile Assisted Hard Handoffs (MAHHOs). The assistance provided by the wireless device during a MAHHO is called a Candidate Frequency Search (CFS). In a MAHHO, as a wireless device approaches the edge of its current coverage area, the BS providing the coverage orders the wireless device to perform a CFS. In performing the CFS, the wireless device will check the power level of the pilot signal for the service frequency in an adjacent coverage area (perform a CFS) and report the results of the CFS back to the BS. If the wireless device reports an adjacent frequency pilot signal sufficient for receiving by the wireless device, the BS will order the wireless device to perform a hard handoff to the adjoining coverage area. If the wireless device reports a pilot signal power level insufficient to be received by the wireless device, the BS may allow the wireless device to stay on its current serving frequency until the wireless device reports pilot signal power levels sufficient to be received by the wireless device in a subsequent CFS.
A MAHHO may reduce the number of calls dropped during hard handoffs and the instances of oscillation between serving frequencies. However, while the wireless device is performing a CFS, the wireless device may also be generating voice packets for transmission or the BS may be transmitting voice packets for receipt by the wireless device. Unfortunately, a MAHHO requires the wireless device to tune away from the serving frequency during a CFS. Therefore, the above-mentioned voice packets are lost as a consequence of the CFS. The loss of these packets can result in a noticeable degradation of the voice communications.
It would be advantageous if a CFS for a wireless communications device could be performed with a minimal disruption of transmit and receive voice packets.